/**
 * Created with IntelliJ IDEA
 * Description:
 * User: SEKI TOMOKI
 * Date: 2023-10-07
 * Time: 15:13
 */
/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public boolean hasCycle(ListNode head) {
        ListNode fast = head;
        ListNode slow = head;

        //快指针走两步，慢指针走一步，如果有环就一定相遇
        while(fast != null && fast.next != null) {
            fast = fast.next.next;
            slow = slow.next;
            if(fast == slow) {
                return true;
            }
        }

        return false;
    }
}
